In ultrasonic cleaning, a transducer, usually piezoelectric but sometimes magnetostrictive, is secured to or immersed in a cleaning tank to controllably impart ultrasonic vibration to the tank. The tank is filled with a cleaning liquid and parts are immersed into the liquid to be cleaned by ultrasonic agitation and cavitation. The ultrasonic energy itself can dislodge contaminants. Under certain conditions the ultrasonic energy also creates cavitation bubbles within the liquid where the sound pressure exceeds the liquid vapor pressure. When the cavitation bubbles collapse, the interaction between the ultrasonically agitated liquid and the contaminants on the parts immersed in the liquid causes the contaminants to be dislodged.
Various circuits have been configured for driving ultrasonic transducers and have provided a variety of features. Parameters which are available for adjustment or control are the ultrasonic frequency, the power level, amplitude or frequency modulation, and duty cycle control of power bursts, among others.
In ultrasonic cleaning, it is known that the output circuit, which usually includes a driver, the ultrasonic transducer, and the load have a resonant frequency. The load, of course, includes the cleaning tank, the liquid in the tank, and the parts immersed in the liquid. Quite clearly, the mass and shape of the parts, the temperature of the liquid, and other factors all influence the resonant frequency of the output circuit.
It is known that when the ultrasonic transducer is driven at the resonant frequency of the load, the system is capable of delivering maximum power to the load. Cavitation of the cleaning liquid can be enhanced by modulating the driving frequency, which implies moving away from resonance. Because the phase angle changes as the generator is modulated away from the resonant frequency, this dictates that even if the center frequency of the generator is tuned to resonance, as the output frequency is modulated, the real power delivered to the output is reduced. In generators without power control circuitry, the output power will thus fluctuate along with the output frequency as the output frequency is modulated.
Ultrasonic generators with power control capabilities are known in the art. For example, U.S. Pat. No. 5,276,376 to Puskas teaches a generator with power control circuitry capable of maintaining substantially constant power as the phase angle changes. To maintain constant power to the load, however, these systems sought to limit the maximum change in phase angle by limiting the modulation frequency bandwidth to substantially near resonant frequency, e.g., within .+-.1 kHz of resonant frequency. In systems of this type, therefore, a resonant follower automatically tuned the output frequency to resonance, and the modulation range was limited, so that the automatic power control worked within a limited band.
High efficiency is desirable in an ultrasonic generator, not only because high efficiency is generally considered to be more favorable than low efficiency, but also because it allows the components to be sized to match the designed task. Thus, if one were to produce a generator with a 500 watt output, and efficiency could be maintained within say 20%, then no part of the circuit would need to be designed to handle much more than about 600 watts. However, if the phase angle swings are such that the efficiency might vary by as much 2 to 1 or more, and it is desired to have 500 watts out at the worst case conditions, then it might be necessary to have input circuitry capable of handling 1000 watts or more. This extra capacity needed at the input in order to accommodate poor power factors at the output can be considered "head room". Large head room would not be necessary if the phase angle changes were small, but becomes more necessary in order to maintain the output power level as the phase angle changes become larger. Typically it is more straightforward to configure a generator which operates substantially near resonant frequency with perhaps a limited modulation bandwidth, than to configure a generator which might or might not operate at resonance, or may swing through resonance at unpredictable points, and to provide sufficient head room to maintain approximately consistent output power under these possible operating conditions.
In operation a further disadvantage of present ultrasonic generators is that they are incapable of adequately cavitating semi-aqueous cleaning solutions to obtain optimal cleaning results.